This application claims the Paris convention priority of Japanese Patent Application No. 2000-081535 filed on Mar. 23, 2000, which is incorporated herein by reference.
The present invention relates to a nickel-metal hydride storage battery.
Recently, a nickel-metal hydride storage battery having larger capacity than a conventional nickel-cadmium storage battery is regarded as a promising power supply for portable equipment. A nickel-metal hydride storage battery is designed to have negative electrode capacity larger than positive electrode capacity for the purpose of suppressing degradation through oxidation of the negative electrode active material and increase of the battery internal pressure in overcharge.
In accordance with recent development in performance of portable equipment, a nickel-metal hydride storage battery is required to have larger capacity and a longer service life.
For example, a conventionally proposed nickel-metal hydride storage battery with large capacity uses a positive electrode active material of nickel hydroxide including, as a solid-solution element, at least one element selected from the group consisting of Cd, Ca, Zn, Mg, Fe, Co and Mn in a ratio of 1 through 7 wt %, and a negative electrode active material of a hydrogen-absorbing alloy having a CaCu5-type crystal structure (such as MmNi3.6Co0.7Mn0.4Al0.3) (as is disclosed in Japanese Laid-Open Patent Publication No. 5-21064/1993).
In the conventional battery disclosed in Japanese Laid-Open Patent Publication No. 5-21064/1993, since the specific capacity (capacity per unit volume) of the positive electrode active material is large, the battery capacity can be increased at the initial stage of charge-discharge cycles by increasing the density of packing the positive electrode active material. The specific capacity of the negative electrode active material is, however, small, and hence, the ratio of the negative electrode capacity to the positive electrode capacity is extremely small. Accordingly, when charge-discharge cycles are repeated, the negative electrode active material is degraded through oxidation in a small number of cycles, so as to lower the negative electrode capacity. As a result, hydrogen is generated on the negative electrode in overcharge, which causes leakage. In other words, this conventional battery has a problem of a short cycle life. Although this problem can be overcome by increasing the ratio of the negative electrode capacity to the positive electrode capacity, the density of packing the positive electrode active material should be decreased for increasing the ratio. When the density is decreased, the positive electrode capacity is lowered, resulting in lowering the battery capacity.
On the other hand, for example, a conventionally proposed nickel-metal hydride storage battery with a long life uses nickel hydroxide as a positive electrode active material and a hydrogen-absorbing alloy represented by a composition formula, TixVyNiz(wherein 15xe2x89xa6xxe2x89xa625; 50xe2x89xa6yxe2x89xa690; 5xe2x89xa6zxe2x89xa620; and x+y+z=100) as a negative electrode active material (as is disclosed in Japanese Laid-Open Patent Publication No. 6-228699/1994).
In the conventional battery disclosed in Japanese Laid-Open Patent Publication No. 6-228699/1994, since the specific capacity of the negative electrode active material is larger than that of the aforementioned conventional battery, the ratio of the negative electrode capacity to the positive electrode capacity can be sufficiently increased by decreasing the density of packing the positive electrode active material. Thus, the cycle life of the battery is increased. Since the specific capacity of the positive electrode active material is smaller than that of the aforementioned conventional battery, however, the positive electrode capacity is extremely small. In other words, this conventional battery has a problem of small battery capacity. Although this problem can be overcome by increasing the density of packing the positive electrode active material so as to increase the positive electrode capacity, the density of packing the negative electrode active material should be decreased for this purpose. When the density of packing the negative electrode active material is decreased, the ratio of the negative electrode capacity to the positive electrode capacity is lowered, resulting in shortening the cycle life.
The present invention was devised to overcome the aforementioned antinomic problems, and an object of the invention is providing a nickel-metal hydride storage battery having large battery capacity and a long cycle life.
The nickel-metal hydride storage battery of this invention (present battery) comprises a nonsintered nickel electrode using, as a positive electrode active material, nickel hydroxide including, as a solid-solution element, at least one element Q selected from the group consisting of Mn, Al, Y, Yb and Co; and a pasted hydrogen-absorbing alloy electrode using, as a negative electrode active material, a hydrogen-absorbing alloy represented by a composition formula, TiaVbNicMd, where in a+b+c=100; 15xe2x89xa6axe2x89xa645, 35xe2x89xa6bxe2x89xa675; 5xe2x89xa6cxe2x89xa625; 0 less than dxe2x89xa67; and M is at least one element selected from the group consisting of Cr, Mn, Mo, Nb, Ta, W, La, Ce, Y, Mm, Co, Fe, Cu, Si, Al, B, Zr and Hf, and the capacity ratio between the nonsintered nickel electrode and the pasted hydrogen-absorbing alloy electrode is 1:1.1 through 1:1.8.
As a result, the nickel-metal hydride storage battery of this invention can attain large battery capacity and a long cycle life.